Spawning induction in fish and GnRH regulation of gonadotropins: Modes of action

Improvement of the hypophysation technique in Israeli aquaculture has been achieved by introducing LH-calibrated carp pituitary extract (CCPE) as a spawning-inducing agent. The hypothalamic approach introduced in the 90s combines GnRH analog and a water-soluble dopamine antagonist in a single injection. Profiles of LH, estradiol and 17α, 20 β dihydroxy-4-pregnen-3-one (17,20-P) and the progress in oocyte maturation determined in treated carp revealed how the two approaches reach the same end result. The transduction pathways of GnRH signal leading to LH release in fish were found to encompass the activation of Gq protein, PLC, phosphoinositide turnover, formation of IP₃ and diacylglycerol, Ca²⁺ influx and mobilization, and activation of PKC and PKA. Recent studies indicate that GnRH signal is further transduced to the nucleus via PKC and PKA converging at MAPK (extracellular signal- regulated kinase=ERK) to stimulate the expression of the common α and the LH β subunit genes, while that of the FSH β is transduced via PKA in a manner independent of ERK.